1. Field of the Invention
The present invention relates to data communications equipment. More particularly, the present invention relates to translation tables and constellation designs for a PCM modem and methods of generating the same.
2. State of the Art
With the ever-increasing importance of telecommunications for the transfer of data as well as voice, there has been a strong effort to increase data transfer rates over the telephone wires. In 1994, the ITU-T adopted the V.34 Recommendation (International Telecommunication Union, Telecommunication Standardization Sector Recommendation V.34, Geneva, Switzerland 1994). The V.34 standard and subsequent amendments define modem operating speeds of 28.8 kbps up to 33.6 kbps, and the vast majority of modems being sold today adhere to the V.34 Recommendation. However, with the explosion in the use of the Internet, even at the V.34 transfer rates, downloading of large files available on the Internet can take long periods of time. Thus, even as the V.34 standard was being adopted, there was a thrust to provide additional standards recommendations which would increase data transfer rates even further.
Recognizing that further increases in data rates is theoretically limited where the telecommunication network is an analog system (see C. E. Shannon, xe2x80x9cA Mathematical Theory of Communication,xe2x80x9d Bell System Technical Journal, 27:379-423, 623-656 (1948)), there have been various proposals to take advantage of the fact that much of the telecommunication network is now digital. For example, U.S. Pat. No. 5,394,437 to Ayanoglu et al., U.S. Pat. No. 5,406,583 to Dagdeviren, and U.S. Pat. No. 5,528,625 to Ayanoglu et al. (all assigned to ATandT/Lucent and all of which are hereby incorporated by reference herein in their entireties) all discuss techniques which utilize the recognition that the network is mostly digital in order to increase data transmission rates to 56 kbps and higher. Similarly, Kalet et al., xe2x80x9cThe Capacity of PAM Voiceband Channels,xe2x80x9d IEEE International Conference on Communications ""93, pages 507-511 Geneva, Switzerland (1993) discusses such a system where the transmitting end selects precise analog levels and timing such that the analog to digital conversion which occurs in the central office may be achieved with no quantization error. PCT application number PCT/US95/15924 (Publication WO 96/18261) to Townshend which is hereby incorporated by reference herein in its entirety) discusses similar techniques. All of the disclosures assume the use of PAM (pulse amplitude modulation) digital encoding technology rather than the QAM (quadrature amplitude modulation) currently used in the V.34 Recommendation. The primary difference between the ATandT technology and the Townshend reference is that the ATandT technology suggests exploiting the digital aspect of the telephone network in both xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d directions, while Townshend appears to be concerned with the downstream direction only. Thus, systems such as the xe2x80x9cx2xe2x80x9d technology of US Robotics which are ostensibly based on Townshend envision the use of the V.34 Recommendation technology for upstream communications.
Recently, a new Recommendation for standard was adopted by the ITU-T for the purposes of standardizing a PCM-type modem. The new standard, known as xe2x80x9cV.90xe2x80x9d, which is hereby incorporated by reference herein in its entirety, relates primarily to the transmitter of a PCM-type modem. In Section 8.4.5, the V.90 Standard requires a two-level training signal TRN1d which is utilized for modem equilization. In Section 8.4.1, the V.90 Standard requires the provision of a probing signal; also known in the art as digital impairment learning or xe2x80x9cDILxe2x80x9d. The purpose of the DIL is to give the receiver of the receiving (analog) modem the opportunity to measure network impairments. The measurements and determinations made by the receiving modem are used by the receiving modem in formulating an appropriate constellation for the transfer of data. The constellation formulated by the receiving modem is transmitted back to the transmitting modem as a DIL descriptor set forth in Section 8.3.1 of the V.90 standard.
While much attention has been paid in the prior art to the transmitter in the V.90 modem, it will be appreciated that ability to design an appropriate transmission constellation plays a critical role in producing a high quality modem. In particular, according to V.90, the transmitter transmits 8-bit binary numbers (octets) which correspond to 128 positive and 128 negative xcexc-law or A-law levels. These octets go through the digital network and are finally transformed into analog levels in a digital-to-analog (D/A) converter in the central office. To maximize data rates in the presence of network impairments, an optimal signal constellation must be utilized. Thus, it is necessary to relate (correspond) the transmitted octets to the levels received at the D/A output. This relation or correspondence is accomplished by reference to a translation table. Determination of the translation table is not a trivial task because the digital channel has uncertain parameters and the PCM signal is subjected to both digital and analog distortions including digital attenuation (PAD), robbed bits, etc. In fact, even the distortion may not be regular from frame to frame, as there exists a robbed bit signaling convention known as alternating robbed bit signaling (ARBS) where a slot is subjected to robbed bit signaling one, two or three times during four frames (alternating robbed bit signaling having a four frame periodicity). Regardless, preparation of an appropriate translation table is critical to the high-quality functioning of the data communications. In addition, the translation table is necessary for generating an appropriate constellation design.
It is therefore an object of the invention to provide methods for generating translation tables in the receivers of PCM modems, and apparatus which utilize the methods.
It is another object of the invention to provide methods for generating translation tables in the receivers of PCM modems where account is made for digital and analog impairments, and in particular account is made for alternating robbed bit signaling.
If is a further object of the invention to provide methods of designing one or more constellations based on translation table determinations.
In accord with the objects of the invention, the receiver of PCM modem of the invention utilizes the V.90 TRN1d training signal for detecting whether any of the six slots of the received signal may be subject to alternating robbed bit signaling (ARBS), and what the alternating robbed bit signaling pattern may be. This is preferably accomplished by accumulating the received level over a plurality of frames for each slot and by comparing the received levels for each slot to an average level for that slot. If the difference between the received levels and the average levels exceeds a threshold, the slot is determined to be subject to ARBS.
According to another aspect of the invention, where a slot is subject to ARBS, DIL sequence signals for frames of the alternating robbed bit signaling slot having LSB=0 and LSB=1 are accumulated separately in order to generate two translation tables (TRT0, TRT1) for that slot. Where the DIL sequence is found only in the frames having the alternating robbed bit signaling slot having LSB=0 or having LSB=1, only one translation table can be generated.
According to a further aspect of the invention, different algorithms are provided for designing a constellation for the alternating robbed bit signaling slot depending on (a) whether two translation tables were generated for the slot, (b) whether the frame-to-TRT correspondence is known, and (c) where only one translation table is generated, whether the translation table generated was TRT0 or TRT1. Where two translation tables were generated, and the frame-to-TRT correspondence is known, constellation points are selected utilizing a minimum distance (dmin) between levels of table TRT1. The Ucodes for the selected levels in TRT1 are used to identify selected levels from table TRT0. Constellation points are selected until the average power of the transmitted signal for the alternating robbed bit signaling slot (including both sets of selected levels) exceeds a given threshold. As a result, a single set of Ucodes for the alternating robbed bit slot is provided for the transmitting modem, while two sets of references are provided for the receiving modem.
Where two translation table were generated, but the frame-to-TRT correspondence is not known, a first constellation point is selected from table TRT1 utilizing the required minimum distance (dmin) between levels of table TRT1. The Ucode for the selected level in TRT1 is used to identify a selected level from table TRT0. The next constellation point in TRT is selected by finding the next constellation point in table TRT1 a minimum distance from the previous selected level in table TRT0. The Ucode for that next selected point is used to identify another selected level from table TRT0. Additional constellation points are selected in this manner until the average power of the transmitted signal for the alternating robbed bit signaling slot (including both sets of selected levels) exceeds a given threshold. As a result, a single set of Ucodes for the alternating robbed bit slot is provided for the transmitting modem, while two sets of references are provided for the receiving modem. However, since the two sets of references cannot be correlated to the received frames, a double decision is required by the receiving modem.
Where only a single translation table is generated, a constellation is designed using only that translation table. The result is a set of Ucodes for the transmitter and a first set of references for the receiver. Then, for each level from the first set of references, the same level is found in a xcexc-law table. If the translation table available is TRT0, the next smaller level in the xcexc-law table is taken as a point for the second set of references; whereas, if the translation table available is TRT1, the next larger level in the xcexc-law table is taken as a point for the second set of references. The selection of the constellation levels continues until the average power of transmitted signals for the alternating robbed bit slot exceeds a given threshold.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.